I. Field of the Invention
This invention relates to high strength alumina materials and processes for producing the same.
II. Description of the Prior Art
Alumina, or aluminum oxide (Al.sub.2 O.sub.3), is a commonly used ceramic material. It may be useful in applications where light, high strength material is required. For example, it may be useful for making high strength parts, tools, armor and implants. It is usually desirable in such applications to utilize materials having high compressive strength, flexure strength, impact strength and wear resistance.
Alumina powders used to make alumina materials typically include an appreciable amount of other materials, For example, Ceraver, a brand of commercially available alumina material, is only 92% alumina. Ordinarily, the presence of such impurities adversely affects the physical properties of the alumina material.
The formation of ceramic materials from alumina powder typically requires processing at elevated temperatures. An undesirable effect of such processing is that grains may grow to large, widely distributed sizes. Similarly, such grains may be irregularly shaped. Therefore, sintering agents, such as MgO, are typically added to the alumina powder to control the growth of such grains. However, addition of such sintering agents has the undesirable effect of introducing materials other than alumina into the material so formed. The final material formed by such a process usually includes a glassy phase. This glass phase introduces "weak" areas between the grain boundaries of the sintered material. Therefore, it is desirable to avoid the addition of sintering agents.
During formation of Al.sub.2 O.sub.3 powder into high strength materials, the resulting material will have various microstructures, depending upon the particular processing techniques, conditions and materials employed. It is desirable to control the microstructure formed during processing, so as to produce materials with desirable properties. For example, it is usually desired to produce a material with small grains, and a uniform distribution of grain sizes, because such grain characteristics are commonly associated with high strength.
Similarly, it is desirable to produce an alumina material with uniform, equiaxed, isometric grains, because such grains are commonly associated with high strength. This property maybe measured by a Form Factor, defined as follows: EQU FF=4.pi.A/P.sup.2
where
P=grain perimeter; and PA1 A=cross-sectional area of the grain. A perfectly circular grain would have a form factor of 1.0. Ideal, high density, equiaxed grained ceramic materials in two dimensions, like alumina, can be expected to have six neighboring grains. The theoretical form factor for such structures is 0.91.
It is desirable to produce a material with low porosity, because such low porosity is commonly associated with high strength. It may be desirable to produce a material with predominantly transgranular porosity, rather than intergranular porosity, because such predominantly transgranular porosity may be less detrimental to its strength properties. Transgranular porosity is a characteristic wherein void spaces are contained within individual grains themselves, rather than being located at grain boundaries. Intergranular porosity is a characteristic wherein void spaces are located on grain boundaries, rather than within individual grains.